The present invention relates to spring probe block assemblies of the type used in Automatic Test Equipment (ATE), and specifically to spring probe block assemblies for use in high bandwidth applications.
Spring probe blocks are used to provide temporary spring contact interfaces between integrated circuits or other electronic equipment and the automated test equipment test head to run necessary tests of integrated circuits or other electronic equipment. Spring probe block assemblies of the type used in automatic test equipment are widely available and use generally similar designs. Spring probe block housings are typically machined from metal bar stock in a costly sequence of processes that assure precise location and diameter of the bores that accept press fitted coaxial probes and ground receptacles. The solid metal fabrication also serves to commonly ground all of the circuit elements, which until recently was considered desirable from a signal integrity perspective. Some spring probe block housings have also been made of a molded polymer instead of a machined metal.
With both the metal and polymer probe block housings, coaxial probe connectors are individually terminated to coaxial cables at one end and to spring probes at the other. Typically, one spring probe is provided for each signal line, and one or more spring probes are provided to serve as a reference (ground) for each signal line. In the case of polymer spring probe housings, coaxial shield tubes and ground spring probes associated with each signal line can be electrically isolated from their neighbors by the dielectric material of the polymer housing. This isolation of each channel (consisting of a signal line plus its associated ground return loop) is necessary to achieve higher bandwidths. The ability to work at high bandwidths is important because the next generation of automated test equipment will be used not only to test faster integrated circuits, but also to test integrated circuits more quickly.
Many currently available spring probe block assemblies are not suitable for use in high bandwidth applications because their designs suffer from one or more infirmities. In particular, many of the prior art spring probe block assemblies (specifically those made using a metal housing) provide a common ground for all of the ground probes. As discussed above, common grounding is not suitable for high bandwidth applications. Rather, for high bandwidth applications it is desired to have the signal probe and its associated ground probes electrically isolated from other coaxial signal and ground probes.
Many of the prior art designs (those using both metal and polymer housings) are also unsuitable for use in high bandwidth applications because of the presence of excessively large ground return loops. FIG. 1A shows a prior art spring probe block assembly 10 that utilizes a polymer housing 12. The ground probes 14 and the signal probe 16 are inserted through holes 18 in the front of the polymer housing 12, with the ground probes 14 being received by box contacts 20. Box contacts 20 are soldered to the coaxial connector 22, which terminates coaxial cable 23 and receives the signal probe 16.
As is illustrated in FIG. 1B, the excessive length of the ground loop (illustrated by dashed line 30) limits the bandwidth because of increased inductance. The ground loop 30 runs from the tip of signal probe 16, through ground probe 14 into box contact 20, along beams 32 of box contact 20, through the weld 34 and then along the conductive shield 36 of the coaxial connector 22. The length of the ground loop is worsened by the thickness of the polymer housing 12 through which the signal and ground probes 16, 14 must pass.
It is well known that at high speeds, the inductance of a given return current path is far more significant than its resistance. In fact, high-speed return currents follow the path of least inductance, not the path of least resistance. Further, it is well know that the lowest inductance return path lies directly under a signal conductor. This means that minimizing the total ground loop length between the outgoing and returning current paths will lead to the lowest possible inductance. Thus, in FIG. 1B, an ideal ground loop is illustrated by dashed line 38. (See High Speed Digital Design: A Handbook of Black Magic by Howard Johnson and Martin Graham).
In addition to the above infirmities, many available designs of spring probe block assemblies require additional components or manufacturing steps to retain the ground spring probe in the assembly. In some instances, tubular receptacles for receiving and retaining the ground spring probes are used. For example, as shown in FIG. 2, in a metal spring probe block housing 40, after a bore 42 is machined into the housing 40 a tubular metal receptacle 44 is press fit into the bore 42, and then the ground spring probe 46 is inserted into the receptacle 44 where it is held in place by a press fit. The receptacle 44 is used to add compliance to the system and avoid damage to the ground spring probe 46, because the ground spring probe 46 itself has very little compliance. The use of probe receptacles 44 adds the undesirable requirements of additional assembly steps and additional parts to be inventoried. In other instances where a tubular receptacle is not used, the ground spring probe is manufactured with what is referred to as a xe2x80x9cbanana bendxe2x80x9d. The banana bend allows the ground spring probe to be inserted into an oversized bore and retained within the bore by a frictional fit. However, manufacturing a spring probe with a banana bend is difficult and costly, and requires that different types of spring probes be used for the signal and ground lines. Clearly, the added manufacturing difficulty and cost, as well as the increased inventory is undesirable. In both of the above described situations, replacing a damaged ground spring probe if very difficult without damaging the remainder of the assembly.
Clearly, what is needed is a spring probe block assembly that can provide a cost effective approach for providing electrically stable, low inductance paths between coaxial connectors and their ground probes. Preferably, such a spring probe block assembly would eliminate the need for ground probe receptacles (and their associated cost, assembly labor, and longer impedance path). In addition, the spring probe block assembly would not require the used of a ground spring probe having a banana bend when no ground probe receptacle is used. Preferably, the spring probe block assembly would also facilitate the replacement of spring probes and coaxial connectors within the block assembly without requiring extensive rework or even scrapping of the entire spring probe block assembly. In addition, the spring probe block assembly would preferably be resistant to high cable pullout forces that could inadvertently dislodge the coaxial connectors during motion of the automated test equipment.